1. Field of the Invention
This invention relates to a node unit used in a digital signal transmission system that transmits a transmission signal with a frame structure and to a state notice information acquiring method used in the node unit. More particularly, this invention relates to an improvement in a method of determining whether to carry out a control function on the basis of the state notice information by determining whether the state notice information transmitted in synchronization with the transmission signal is valid or invalid.
2. Description of the Related Art
In a digital signal transmission system with a plurality of node units, there are provided various protective functions for the purpose of preventing unnecessary responses to bit errors occurring sporadically and thereby stabilizing the operation of the system. One example of the protective functions is the number of frame protecting stages for protecting the frame synchronization of the transmission signal. Some system operation information transmitted in synchronization with the transmission signal has the number of protecting stages set in it. In the explanation below, state notice information used in the autonomous operation of a node unit will be taken as an example of system operation information.
The protecting operation in a conventional system will be explained by reference to FIG. 1. When a failure, such as the cutoff of the signal, occurs at time X, a signal with a specific bit pattern (assumed to be fixed pattern (A)) is transmitted as an alarm from a unit on the upstream side. The signal uses alternating data, such as 010101 . . . , and, in many cases, has no frame structure. Although a node which have received this signal might lose track of the end position of the transmission signal frame because the signal does not have a frame structure, the node attempts to hold on until the protecting period of five stages of frames has elapsed. Then, when the alarm still lasts after the frame protecting period has elapsed, the node senses Out of Frame, or the collapse of the frame synchronization.
On the other hand, for example, the protecting period for three stages of frames is set in the state notice information received at the node. That is, after the signal has been disconnected, the state notice information during the normal operation is held for up to three frames. When a change in the state notice information is sensed after the three frames have passed, an interrupt signal for taking in the state notice information is generated, and the contents of the register which holds the state notice information are taken in immediately. Then, according to the taken-in contents, an autonomous control operation is started at the node.
After the signal has been disconnected, the bit pattern of the state notice information is replaced with the bit pattern of the alarm. Thus, as shown in FIG. 1, the bit string of fixed pattern (A) has been written in the state notice information at the time when the interrupt occurred. More precisely, the bit string obtained by descrambling the bit string 010101 . . . of fixed pattern (A) has been written in the state notice information.
In a conventional system, when an interrupt has occurred, the state notice information is taken in immediately. As a result, the bit string based on the alarm might be mistaken for the normal state notice information, which might therefore permit an erroneous control operation to be performed.
As described above, in the conventional digital signal transmission system, when a node has sensed a change point in the state notice information protected for an n number of stages, this state notice information is taken in immediately and control according to the contents of the information is started. When m-stage protection is applied to the transmission signal and the function of notifying a downstream-side unit of the presence or absence of a failure on the upstream side by replacing the bit pattern of the transmission signal is provided, the replaced bit pattern is taken in as state notice information, if the expression n≦m holds. That is, there is a possibility that the process based on the erroneously recognized state notice information will be carried out and therefore the node will operate erroneously.